1. Field of the Invention
The present invention relates to an electric motor, and more particularly to a structure of a rotor assembly of a permanent magnet motor to improve the motor performance and reduce torque ripple.
2. Description of the Relevant Art
The motor is a well-known electrical machine that converts electrical energy into mechanical energy using magnetic field linkage. Permanent magnet (‘PM’) electric motors are known for their high efficiency. In recent years, especially since the introduction of rare-earth magnetic materials, PM motors have become popular since they eliminate the need for commutator and brushes, which are commonly used with conventional DC electric motors. The absence of the external rotor excitation eliminates losses on the rotor and makes PM motors more efficient. It is well known that electrical motors equipped with commutator and brushes are susceptible to higher maintenance costs. The brushless design of the PM motors allows the conductor coils to be located in a stationary motor stator, which reacts to movement of a rotor that includes the PM's.
PM motors are also known for their durability, controllability, and absence of electrical sparking. Thanks to their advantages the PM motors are widely used in such applications as electric vehicles and servo applications. However, one of the well-known problems of the PM motors is torque ripple. Sources of torque ripple in the PM motors include inherent harmonics in the flux linkage, and cogging. Cogging is a term used to describe the torque of a PM motor disconnected from the power source. Presence of harmonics in the flux linkage results in instantaneous torque that pulsates as a function of the rotor position.
Torque ripple is generally undesirable in many PM motors applications, particularly at low speeds. In the past many techniques have been proposed to minimize torque ripple. Some of these techniques are described in further detail in the following U.S. patents, which are hereby incorporated herein by reference into this specification: 1) U.S. Pat. No. 6,462,452 to Nakano, et al., entitled ‘Permanent magnet motor’, 2) U.S. Pat. No. 5,886,440 to Hasebe, et al., entitled ‘Electric motor with plural rotor portions having pole members of different widths’, and 3) U.S. Pat. No. 6,498,449 to Chen, et al. entitled ‘Low ripple torque control of a permanent magnet motor without using current sensors’.
However, many of the techniques describe a controls based solution to ripple torque reduction. This generally results in the deployment of additional control circuits and/or sensors to reduce the torque ripple. The additional controls add to the complexity and cost. Some other techniques describe a skewing arrangement between the stator and the rotor to reduce the ripple torque. For example, the skewing techniques described by Nakano provide a skew having an electric angle of 72 degree relatively between the rotor and the stator to reduce the fifth harmonic. This results in targeting the reduction of a specific harmonic element in the torque ripple. Additionally, the exact skewing arrangement between the rotor and stator generally results in a higher complexity in design and a higher manufacturing cost.
Thus, a need exists to provide an improved PM motors that offers reduced torque ripple. Specifically, it would be desirable for the improved PM motor to be cost effective, be easy to manufacture and be applicable to any stator design.